This invention relates to the production of articles of expanded thermoplastic polymer material. Such articles are manufactured from the thermoplastic material which is provided in particulate form generally referred to as beads. The beads are made expandable during their manufacture by incorporating a so called blowing agent within. Because of their generally good flowing properties such expandable polymer beads can be easily charged into a closed mold and accordingly lend themselves readily to the manufacture of these articles by expansion and fusion in closed molds. Upon the application of heat the beads undergo a very great expansion; for example, they expand to about 30 to 40 times their original size. If expandable polymer beads were charged directly to the mold for in situ expansion they would occupy very little space and would be accumulated on the bottom of the mold. Consequently when heat is applied to the mold, the beads, not being agitated, would not expand uniformly, the hotter particles expanding before the cooler particles. This would result in the premature fusion of the earlier softened and expanded beads. The molded article would be poorly fused and contain large voids and particles of widely varying densities. For these reasons it is usually preferred to partially expand or "pre-expand" the particles in an apparatus known as a pre-expander to approximately the desired density of the finished article, fill the mold with the pre-expanded beads and further expand and fuse the pre-expanded beads together to form the finished article by applying heat to the particles in the mold.
The pre-expansion is generally accomplished by heating the beads, which contain a blowing agent comprised of a gas or a composition which upon being heated to its boiling point produces a gas, to a temperature at which the bead walls are softened. The softened bead walls expand in response to the pressure exerted by the gaseous expanding agent contained within them. The process may be carried out by either the batch or continuous methods. A typical process for the manufacture of pre-expanded polymeric particles is disclosed in U.S. Pat. No. 3,023,175, issued on Feb. 27, 1962 to Rodman Jr.
While many of the foamed plastic products commonly in use such as ice chests, floats, etc. are made of low density plastic foam there has especially in recent years developed a need for high density expandable or foam plastic items such as shoe platforms. Thus the inventive method and apparatus disclosed herein is particularly capable of uniformly pre-expanding the commonly available expandable polymer beads to produce high density (on the order of 8-15 pounds per square inch or greater) pre-expanded beads. The pre-expanded beads produced are particularly notable for their uniform density of whatever the desired value and they are dry and flowable and ready to be immediately charged into the mold for molding of the manufactured article.
This is a great advantage over pre-expanders such as disclosed in the aforementioned U.S. Pat. No. 3,023,175 patent which uses steam to heat the beads to pre-expand them in preparation for molding. Such a method not only results in wet beads that must be dried to make them flowable but as described therein this method of pre-expansion results in the requirement for aging the beads for long periods of time in storage bins, or the like, for as long as twenty-four hours prior to their use in molding or irregular foaming will occur in the mold.
The method and apparatus of the present invention eliminates entirely this long aging period and allows one to mold with the pre-expanded beads immediately upon their leaving the pre-expander. Thus the substantial storage facilities and materials handling and conveying apparatus required for aging the beads can all be eliminated with the resultant savings benefit.
As a result of the elimination of steam as the heating medium the pre-expanded beads leave the pre-expander of the present invention substantially dry and in a flowable state so that it will not stick or jam and can be used to fill molds of any desired shape. Further since they may be immediately molded they will retain their heat from their exposure in the pre-expander and it is found that relatively little heat in addition is required to be added in the mold to cause the beads to further expand therein and fuse together. Since the heating cycle can be thus shortened in the mold the cooling cycle is likewise shortened and thus an overall molding cycle time saving of up to about 60 percent may be attained, as an example. The residence time of the beads in the pre-expander is likewise very short and thus substantial time and energy savings are made possible by the present invention.